Bump sphinx from 2.3.1 to 2.4.0

Bumps [sphinx](https://github.com/sphinx-doc/sphinx) from 2.3.1 to 2.4.0.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/2.0/CHANGES)
- [Commits](https://github.com/sphinx-doc/sphinx/compare/v2.3.1...v2.4.0)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

requirements-dev.txt

@@ -5,7 +5,7 @@ pytest==5.3.5
 pytest-cov==2.8.1
 pytest-mock==2.0.0
 coverage==5.0.3
-sphinx==2.3.1
+sphinx==2.4.0
 pep257==0.7.0
 aiohttp-session==2.9.0
 aiopg[sa]==1.0.0